It is known that, in an engine using a three-way catalytic converter for purifying CO, HC and NOx, a high purifying efficiency is obtained If the AFR of the gas mixture supplied to the combustion chamber of the engine is periodically made to vary within a narrow range (referred to as a catalyst window) centered on a theoretical AFR.
This is generally accomplished by a feedback control of the gas mixture. e.g. an injection amount of a fuel injection valve is controlled according to the output of an oxygen sensor (O.sub.2 sensor) installed in the exhaust pipe, and an AFR feedback correction coefficient .alpha. is updated according to tile variation of the AFR about a theoretical AFR.
The output of the O.sub.2 sensor varies sharply when the AFR of the gas mixture supplied to the engine crosses the theoretical AFR from rich to lean or vice versa. Hence, it is possible to detect this AFR variation around the theoretical AFR, and to determine whether the present AFR is rich or lean.
The updating amount of the AFR feedback correction coefficient .alpha. comprises a step fraction and an integral fraction, the step fraction being added to .alpha. when the AFR variation includes the theoretical AFR, and Integral fractions smaller than the step fraction being added when the AFR remains either rich or lean until it again crosses the theoretical AFR. The AFR therefore varies stably and periodically within a predetermined narrow range about tile theoretical AFR as center.
This kind of AFR feedback control is conventionally performed when the engine and catalyst in the catalytic converter are fully warmed up. In Japanese Tokkai Sho 61-241434 and Tokkai Sho 60-209646 published by the Japanese Patent Office, however, such a device is disclosed wherein the feedback control is begun immediately after start-up, i.e. when the engine is still cold.
If the temperature of the engine cooling water is low immediately after start-up, a wall flow is easily set up wherein part of the fuel supplied to the engine flows along the walls of the intake port without being converted to spray. This wall flow leads to a fuel supply response delay so that the AFR tends to lean. To deal with this, this device sets the target AFR more on the rich side than the theoretical AFR when the water temperature is low, and the AFR is controlled in a range centered on this target AFR. The effective operating range of the three-way catalyst is thereby enlarged, and the overall exhaust performance of the engine is improved.
However, this fuel injection response delay leads not only to a shift of AFR. If control is performed using a feedback correction coefficient .alpha. set when the engine is hot, the amplitude of the AFR variation In the catalyst window is less than when the engine is hot, and the conversion efficiency of the three-way catalyst consequently deteriorates by a corresponding amount.
Further, the output of the O.sub.2 sensor installed in the exhaust pipe is affected by the temperature of the exhaust system, and at low temperature, the change-over of the sensor output to lean tends to be more sluggish than its change-over to rich. As a result, if the temperature of the exhaust system is low and the same feedback control is performed as in the hot state, the AFR is shifted to lean, and the optimum exhaust conversion efficiency of the three-way catalyst is not obtained.